As part of the 22.033/22.33 Nuclear Systems Design project, this group designed a
100 kW[subscript e] Martian/Lunar surface reactor system to work for 5 EFPY in support of
extraterrestrial human exploration efforts. The reactor design was optimized over the
following criteria: small mass and size, controllability, launchability/accident safety, and
high reliability. The Martian Surface Reactor was comprised of four main systems: the
core, power conversion system, radiator and shielding.
The core produces 1.2 MW[subscript th] and operates in a fast spectrum. Li heat pipes cool the core
and couple to the power conversion system. The heat pipes compliment the chosen pintype
fuel geometry arranged in a tri-cusp configuration. The reactor fuel is UN (33.1w/o
enriched), the cladding and structural materials in core are Re, and a Hf vessel encases
the core. The reflector is Zr[subscript 3]Si[subscript 2], chosen for its high albedo. Control is achieved by
rotating drums, using a TaB[subscript 2] shutter material. Under a wide range of postulated accident
scenarios, this core remains sub-critical and poses minimal environmental hazards.
The power conversion system consists of three parts: a power conversion unit, a
transmission system and a heat exchanger. The power conversion unit is a series of
cesium thermionic cells, each one wrapped around a core heat pipe. The thermionic
emitter is Re at 1800 K, and the collector is molybdenum at 950 K. These units, operating
at 10[superscript +]% efficiency, produce 125 kW[subscript e] DC and transmit 100 kW[subscript e] AC. The power
transmission system includes 25 separate DC-to-AC converters, transformers to step up
the transmission voltage, and 25 km of 22 gauge copper wire for actual electricity
transmission. The remaining 900 kWth then gets transmitted to the heat pipes of the
radiator via an annular heat pipe heat exchanger that fits over the thermionics. This power
conversion system was designed with much redundancy and high safety margins; the
highest percent power loss due to a single point failure is 4%.
The radiator is a series of potassium heat pipes with carbon-carbon fins attached. For
each core heat pipe there is one radiator heat pipe. The series of heat pipe/fin
combinations form a conical shell around the reactor. There is only a 10 degree
temperature drop between the heat exchanger and radiator surface, making the radiating
temperature 940 K. In the radiator, the maximum cooling loss due to a single point failure
is less than 1%.
The shielding system is a bi-layer shadow shield that covers an 80º arc of the core. The
inner layer of the shield is a boron carbide neutron shield; the outer layer is a tungsten
gamma shield. The tungsten shield is coated with SiC to prevent oxidation in the Martian
atmosphere. At a distance of 11 meters from the reactor, on the shielded side, the
radiation dose falls to an acceptable 2 mrem/hr; on the unshielded side, an exclusion zone
extends to 14 m from the core. The shield is movable to protect crew no matter the initial
orientation of the core.
When combined together, the four systems comprise the MSR. The system is roughly
conical, 4.8 m in diameter and 3 m tall. The total mass of the reactor is 6.5 MT.